1. Field of the Invention
The present invention generally relates to an electrodischarge machining (EDM) device, which is used to machine a work piece by supplying a dielectric liquid to a machining gap formed between a tool electrode and a metal work piece, and applying a voltage pulse to the machining gap, in particular, to an EDM liquid supply apparatus having an ion exchange resin column for maintaining a suitable specific resistance of a water-based dielectric liquid.
2. Description of Related Art
In EDM apparatus, in order to generate an intermittent electrodischarge in the machining gap, a dielectric liquid with a high specific resistance is supplied to a machining gap. A suitable specific resistance of a water-based dielectric liquid (referred as a machining liquid) is generally 5×104 Ω·cm−1×105 Ω·cm. In general, the EDM apparatus is provided with a specific resistance detector and an ion exchange resin column. If the detection value of the specific resistance is lower than a set value, the machining liquid in a storage tank is circulated through the ion exchange resin column.
In many cases, the EDM apparatus is used to fabricate a mold made of hard materials. Such a mold is made of steel or a super-hard alloy. The super-hard alloys refer to alloys formed by sintering powder of metal carbides, especially the well-known super-hard alloy formed by sintering tungsten carbide and cobalt as binder.
During EDM, it is known that electric corrosion may occur to work pieces made of steel or super-hard alloys. The cause of electric corrosion is considered as follows. A corrosion current flows between a tool electrode as a negative pole and the work piece as a positive pole due to the potential difference therebetween, and thus the material of the work piece is dissolved. In order to prevent the undesired electric corrosion, it is known that a machining liquid having a corrosion preventive agent or a rust preventive agent added is used. The machining liquid is disclosed in Patent Document 1 and Patent Document 2.